This invention relates to an ion-conductive polymer electrolyte.
As an ion-conductive polymer electrolyte, there has been known following compounds. For example, an organic polymer electrolyte of polyethylene oxide (PEO) type; an organic polymer electrolyte having a multifunctional polyether molecular structure prepared by a random copolymerization of ethylene oxide portion and propylene oxide portion (Japanese Laid-Open Patent Publication No. 249,361 of 1987); a solid polymer electrolyte comprising an ethylene oxide copolymer containing an ionic compound in dissolved state (Japanese Laid-Open Patent Publication No. 83,249 of 1986); and an ion-conductive polymer electrolyte in which a high polymer solid substance having plasticity is further constituted substantially with a branched-chain of a homopolymer or copolymer which is thermoplastic and has no crosslinkage (Japanese Laid-Open Patent Publication No. 98,480 of 1980).
However, those conventional ion-conductive polymer electrolytes have the following problems.
First, the organic polymer electrolyte of polyethylene oxide type shows a relatively good lithium-ion conductivity in the temperature range of not lower than 40.degree. C., but the characteristic is lowered rapidly at the room temperature range of about 25.degree. C. Accordingly, it is difficult to use the electrolyte for various electric applications.
The organic polymer electrolytes described in Japanese Laid-Open Patent Publication No. 249,361 of 1987 do not show rapid lowering of the lithium-ion conductivity at the room temperature range of about 25.degree. C., but the lowering proceeds at a temperature of not higher than 0.degree. C. which is considered as a practical temperature range. Therefore, a practical ion-conductivity cannot be obtained.
The organic polymer electrolyte described in Japanese Laid-Open Patent Publication No. 83,249 of 1986 is an organic polymer prepared by a random-copolymerization of ethylene oxide with another monomer. The structure of the resulting organic polymer is amorphous as a result of random-copolymerization, but the product quality tends to be unstable since the structure of the organic polymer does not become amorphous sufficiently.
Further, since the organic polymer electrolyte described in Japanese Laid-Open Patent Publication No. 98,480 of 1980 is thermoplastic, a film formed with it is limited only to be simple and a good adhesion of the film to the electrode cannot be obtained.
We, inventors, have earnestly studied ion-conductive polymer electrolytes to solve these problems, and have found that the ion-conductivity can be improved in ion-conductive polymer electrolytes comprising an organic polymer having a specific structure together with a soluble electrolyte salt as shown in Japanese Laid-Open Patent Publications No. 200863 of 1991, No. 200864 of 1991 and No. 200865 of 1991.
Each of the ion-conductive polymer electrolytes disclosed in those Japanese Laid-Open Patent Publications shows an ion-conductivity superior to the aforementioned electrolytes, but the ion-conductivity is gradually lowered with lowering the temperature. Accordingly those electrolytes do not have a satisfactory stability at a low temperature such as -20.degree. C.
The object of the present invention is to solve such problems as described above and to provide an ion-conductive polymer electrolyte which shows an excellent and stable ion conductivity at low temperature such as -20.degree. C. and can be easily handled.